Field of the Invention
The invention is in the field of nanotechnology. Provided are highly luminescent nanostructures, particularly highly luminescent quantum dots, comprising a nanocrystal core of InP and shell layers of GaP and AlP. The nanostructures may have an additional shell layer. Also provided are methods of preparing the nanostructures, films comprising the nanostructure and devices comprising the nanostructures.
Background Art
Semiconductor nanostructures can be incorporated into a variety of electronic and optical devices. The electrical and optical properties of such nanostructures vary, e.g., depending on their composition, shape, and size. For example, size-tunable properties of semiconductor nanoparticles are of great interest for applications such as light emitting diodes (LEDs), lasers, and biomedical labeling. Highly luminescent nanostructures are particularly desirable for such applications.
To exploit the full potential of nanostructures in applications such as LEDs and displays, the nanostructures need to simultaneously meet five criteria: narrow and symmetric emission spectra, high photoluminescence (PL) quantum yields (QYs), high optical stability, eco-friendly materials, and low-cost methods for mass production. Most previous studies on highly emissive and color-tunable quantum dots have concentrated on materials containing cadmium, mercury, or lead. Wang, A., et al., Nanoscale 7:2951-2959 (2015). But, there are increasing concerns that toxic materials such as cadmium, mercury, or lead would pose serious threats to human health and the environment and the European Union's Restriction of Hazardous Substances rules ban any consumer electronics containing more than trace amounts of these materials. Therefore, there is a need to produce materials that are free of cadmium, mercury, and lead for the production of LEDs and displays.
Cadmium-free quantum dots based on indium phosphide are thought to be inherently less stable than the prototypic cadmium selenide quantum dots. The higher valence and conduction band energy levels make InP quantum dots more susceptible to photooxidation by electron transfer from an excited quantum dot to oxygen, as well as more susceptible to photoluminescence quenching by electron-donating agents such as amines or thiols which can refill the hole states of excited quantum dots and thus suppress radiative recombination of excitons.
Inorganic shell coatings on quantum dots are a universal approach to tailoring their electronic structure. Additionally, deposition of an inorganic shell can produce more robust particles by passivation of surface defects. Ziegler, J., et al., Adv. Mater. 20:4068-4073 (2008). For example, shells of wider band gap semiconductor materials such as ZnS can be deposited on a core with a narrower band gap—such as CdSe or InP—to afford structures in which excitons are confined within the core. This approach increases the probability of radiative recombination and makes it possible to synthesize very efficient quantum dots with quantum yields close to unity and thin shell coatings.
Most quantum dots do not retain their originally high quantum yield after continuous exposure to excitation photons. Elaborate shelling engineering such as the formation of multiple shells and thick shells—wherein the carrier wave functions in the core become distant from the surface of the quantum dot—have been effective in mitigating the photoinduced quantum dot deterioration. Furthermore, it has been found that the photodegradation of quantum dots can be retarded by encasing them with an oxide—physically isolating the quantum dot surface from their environment. Jo, J.-H., et al., J. Alloys Compd. 647:6-13 (2015).
The interfaces in these heterogeneous nanostructures need to be free of defects because defects act as trap sites for charge carrier sites and result in a deterioration of both luminescence efficiency and stability. Due to the naturally different lattice spacings of these conductor materials, the crystal lattices at the interface will be strained. The energy burden of this strain is compensated by the favorable epitaxial alignment of thin layers, but for thicker layers the shell material relaxes to its natural lattice—creating misalignment and defects at the interface. There is an inherent tradeoff between adding more shell material and maintaining the quality of the material. Therefore, a need exists to find a suitable shell composition that overcomes these problems. Smith, A., et al., Acc. Chem. Res. 43: 190-200.
Recent advances have made it possible to obtain highly luminescent plain core nanocrystals. But, the synthesis of these plain core nanocrystals has shown stability and processability problems and it is likely that these problems may be intrinsic to plain core nanocrystals. Thus, core/shell nanocrystals are preferred when the nanocrystals must undergo complicated chemical treatments—such as for biomedical applications—or when the nanocrystals require constant excitation as with LEDs and lasers. See Li, J. J., et al., J. Am. Chem. Soc. 125:12567-12575 (2003).
There are two critical issues that must be considered to control the size distribution during the growth of shell materials: (1) the elimination of the homogenous nucleation of the shell materials; and (2) homogenous monolayer growth of shell precursors to all core nanocrystals in solution to yield shell layers with equal thickness around each core nanocrystal. Successive ion layer adsorption and reaction (SILAR) was originally developed for the deposition of thin films on solid substrates from solution baths and has been introduced as a technique for the growth of high-quality core/shell nanocrystals of compound semiconductors.
Kim et al., J. Am. Chem. Soc. 134:3804-3809 (2012) reported the preparation of highly luminescent InP/GaP/ZnS nanocrystals. The intermediate InP/GaP nanocrystals were prepared by a process comprising treating a ZnInP core with GaCl3 and oleic acid that reportedly formed a gallium-oleate complex. Kim et al. reported that the resulting blue-shift that occurred could be explained by Ga3+ replacing In3+ ions near the surface, thus causing a reduction in the size of the InP core. However, the Supporting Information showed that the gallium incorporation was not significant (see S2 and S9). And, the Examples herein demonstrate that a reaction comprising GaCl3 and oleic acid does not result in the incorporation of gallium onto an InP core. Instead, the blue-shifting is likely due to the reduction in size of the InP nanocrystals by oleic acid etching.
A need exists to find a method to prepare InP nanocrystals with GaP and AlP shells. The present invention provides methods for preparing InP/GaP and InP/AlP nanocrystals.